Method and apparatus for controlling output voltages and frequencies of turbine/alternator on common shaft

ABSTRACT

An electrical system and method for a turbine/alternator comprising a gas driven turbine and a permanent magnet alternator rotating on a common shaft includes an inverter circuit connectable either to an output circuit or the stator winding of the alternator. A control circuit during a start-up mode switches the inverter circuit to the stator winding of the alternator and during a power out mode switches the inverter circuit to the output circuit and power line. During the power output mode the power line is monitored and the output voltage, frequency and/or phase is matched or synchronized to the power line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/466,386 filed May 28, 2003 which is a continuation of U.S. patentapplication Ser. No. 09/840,572, filed Apr. 23, 2001, which is acontinuation of U.S. application Ser. No. 09/319,390, filed Jun. 1,1999, which is the U.S. national phase of International Application No.PCT/US97/22405, filed Dec. 3, 1997 which designated, inter alia, theUnited States, and which claims the benefit of U.S. ProvisionalApplication No. 60/032,149, filed Dec. 3, 1996.

BACKGROUND OF THE INVENTION

Gas turbines must be driven to rotate at a starting speed by auxiliarymeans prior to fuel injection and ignition and self-sustained operation.In the past, for example, gear box systems driven by auxiliary electricor compressed air motors have been used to rotate the turbine tostarting speed. “Air” impingement starting systems have also been usedwith small turbines and operated by directing a stream of gas, typicallyair, onto the turbine or compressor wheel to cause rotation of the mainrotor. These prior art systems are complex and difficult to implement.

Electrical power may be generated by using a gas turbine to drive analternator. The alternator may be driven by a free turbine which iscoupled to the rotor of the alternator or through a gear box. In thesesystems, the speed of the turbine must be precisely controlled tomaintain the desired frequency and voltage of the alternating currentoutput.

SUMMARY OF THE INVENTION

In accordance with the present invention, an alternator having apermanent magnet rotor is connected to the main turbine rotor makingpossible both starting of the turbine as well as generation ofelectrical power. The electrical system described herein allows therotor to operate at various speeds with an output frequency and voltageunrelated to rotor speed. The electrical system incorporates a uniqueinverter which yields the appropriate voltage and frequency in both thestart-up mode of operation as well as in the power generation mode ofoperation.

The electrical system is used to cause rotation of the turbine duringthe start-up mode and subsequently is used to extract electrical powerfrom the alternator after the turbine has reached its normal operatingconditions. At start-up, the alternator functions as an electric motor.The functions of the electrical system at start-up comprise power boost,power switching and control to provide, for example, three-phase ACelectrical power to the alternator. Both the frequency and voltage arecontrolled as a function of time and rotation speed. Electrical powerfor the electrical system is obtained during start-up from either a DCsource, such as a battery, or from an AC power line. The start-upcircuit may function as an open loop control system or as a closed loopcontrol system based upon rotor position feedback.

As the turbine approaches normal operating conditions at very highspeeds of rotation powered through the controlled combustion of fuel andair, the electronic circuitry used to initially drive the alternator asa motor is automatically reconfigured to accept power from thealternator. Subsequently, three-phase electrical power becomes availablefor extraction from the electrical system at desired voltages andfrequencies.

Briefly, according to this invention, an electrical system for aturbine/alternator comprises a gas driven turbine and alternatorrotating on a common shaft. The alternator has a permanent magnet rotorand a stator winding. A stator circuit is connected to the statorwinding. A DC bus powers an inverter circuit. The output of the invertercircuit is connected to an AC output circuit or through a firstcontactor to the stator circuit. A rectifier is connected between thestator circuit and the DC bus. A signal generator is driven by signalsderived from the rotation of the common shaft and an open loop waveformgenerator produces waveforms independent of the rotation of the commonshaft. A second contactor connects either the signal generator or theopen loop waveform generator to a driver connected to cause switching ofthe inverter circuit. A temporary power supply supplies energy to the DCbus. A control circuit, during a start-up mode, switches the firstcontactor to connect the inverter circuit to the stator circuit andswitches the second contactor to connect the signal generator to thedriver, preferably a pulse width modulator. The control circuit, duringa power out mode, switches the first contactor to disconnect theinverter from the stator circuit and switches the second contactor toconnect the open loop waveform generator to the driver. During thestart-up mode, the alternator functions as a motor to raise the speed ofthe turbine to a safe ignition speed. The inverter is used to commutatethe stator windings in response to the signal from the signal generator.During a power out mode, the inverter is used to convert the rectifiedoutput of the alternator into AC signals applied to the AC outputcircuit in response to the open loop waveform generator, thus producingelectric power having a frequency unconnected to the rotational speed ofthe alternator.

According to a preferred embodiment, an electrical system for aturbine/alternator comprises a gas driven turbine and alternatorrotating on a common shaft. The alternator is comprised of a permanentmagnet rotor and a stator winding. The stator winding is connectedthrough a contactor to an inverter circuit. The inverter circuit isconnected to a DC bus. The inverter circuit is also connected to asignal generator. A position encoder is connected to the drive shaft ofthe turbine/alternator. Its output is also connected to the signalgenerator. The inverter processes the DC bus voltage and signalgenerator output to develop three-phase AC output voltages. The signalgenerator controls the inverter output frequency. Concurrently, avariable voltage DC power supply applies a time variant voltage to theDC bus. The DC bus voltage controls the inverter output voltage level.Thus, the output frequencies and voltages of the inverter are fullycontrollable. During the start-up mode, the output of the inverter isapplied through a contactor to the alternator which functions as anelectric motor. When the start-up mode is initiated, the DC power supplyvoltage begins to ramp up from 0 volts. The signal generator outputfrequency is set to a fixed low frequency. As the DC bus voltage beginsto increase, the alternator rotor begins to rotate at a low speed. Theencoder senses shaft position changes and sends this information to thesignal generator. The signal generator processes this information andbegins to ramp up its output frequency as a function of engine speed.This increasing frequency is directed to the inverter where it is usedto control the frequency of the inverter output voltage. This controlledprocess results in a time variant inverter output whose frequency andvoltage are applied through a contactor to the alternator. As a result,the alternator functions as a motor and accelerates the speed of theturbine shaft to a value suitable for ignition. Once the turbine hasreached its normal operating speed, the variable voltage power supply isdeactivated. Further, the shaft position encoder signal is disconnectedfrom the signal generator and is replaced by a precision, fixed timebase signal. Subsequently, the alternator AC output voltage is rectifiedand the resulting DC output voltages are applied to the DC bus. Thisreconfiguration permits the inverter to operate as a fixed frequencypower output source independent of turbine rotor speed. In the poweroutput mode, the inverter provides power through output filters. Thefiltered output power is then connected to a contactor which directs itto a set of terminals where it is available for consumer use. A controlsystem integrates operation of the inverter, power supply, signalgenerator and contactors during both the start-up and power output modesof operation. During the power output mode of operation, the controlsystem continuously measures output voltages from the inverter and sendssignals to the signal generator to compensate for output voltagefluctuations caused by varying output load conditions.

According to a preferred embodiment, the signal generator is a pulsewidth modulator. Typically, the stator winding of the alternator is athree-phase winding and the inverter circuit and the AC circuits arethree-phase circuits.

According to a preferred embodiment, the electrical system comprises abattery powered supply circuit including a battery and a boost from 0inverter circuit for outputting to the DC bus a voltage between 0 andthat required by the inverter to power the alternator to safe ignitionspeeds. According to another preferred circuit, the battery poweredsupply circuit comprises a step-down circuit for recharging the batteryand for powering low voltage devices such as fans and pumps from the DCbus during the output mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and other objects and advantages will become clear fromthe following detailed description made with reference to the drawingsin which:

FIG. 1 is a schematic drawing showing the overall relationship of theelectrical system to the gas turbine/alternator;

FIG. 2 is a schematic drawing showing the electrical system forproviding electrical power to the alternator during the start-up modeand for passing power generated to the load during the power out mode;

FIG. 3 schematically illustrates a rectifier circuit for converting thealternator output to a DC current voltage on the DC bus;

FIGS. 4 a and 4 b schematically illustrate the inverter circuitcomprised of six IGBT switches used to commutate the current to thealternator during the start-up mode and to provide three-phase outputduring the power out mode;

FIG. 5 schematically illustrates the open loop waveform generator andclosed loop driver for the inverter circuit;

FIG. 6 illustrates a boost/buck chopper suitable for using battery powerduring the start-up mode to power the DC bus and for charging thebattery from the DC bus during the power out mode; and

FIG. 7 schematically illustrates the entire electrical system includingturbine sensors and turbine controls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the relation between the electrical control system 1,according to this invention, and the power generation system comprisinga gas turbine 2 and an alternator 3. The alternator armature is mountedon a shaft common with the turbine shaft. The electrical control systeminteracts with the power generation system to provide start-up power,engine control, signal processing, battery charging, user interfaces, aswell as power conversion and control for generating user power. Bothstand-alone and line tie operations are facilitated.

Referring now to FIG. 2, the general arrangement of the electrical powercircuits for a turbine generator, according to this invention, isdepicted. A turbine 10 is connected to a permanent magnet (rare earthsamarium-cobalt) alternator 11 by a common shaft 12. The stator ismanufactured using a stack of high quality, low loss, electric sheetsteel laminations. This stack contains a three-phase distributed windingin twelve stator slots with a housing with provision for oil cooling.The performance of the alternator depends upon effective cooling. In thecurrently implemented embodiment, the four pole permanent magnet rotorhas the following dimensions: active length 3.55 inches; diameter undermagnets 1.00 inch; diameter over 1.430 inches; weight of magnets 0.70pounds; rotor weight 1.95 pounds.

The three-phase stator windings of the alternator are connected by an ACbus 14 to a rectifier 15. The output of the rectifier is connected to aDC bus 16. During power generation, that is, the power out mode when theturbine is driving the alternator, the three-phase output on the AC busis rectified by the rectifier providing DC power on the DC bus. The DCpower is applied to an inverter 17. The inverter 17 during the power outmode switches the DC power to provide three-phase output having afrequency unrelated to the rotational speed of the alternator. Thefrequency is controlled by signals from a system controller 18. Theinverter output is filtered by inductors 19 and capacitors 20. Thefiltered three-phase output is passed to a load through an outputcontactor 21 (controlled by the system controller 18 through a relay 22)and output breakers 23.

A current transformer 25 senses output current which is fed back to thesystem controller 18 enabling current limit and power balancing of thethree-phase output.

In order to start the turbine, it is necessary to accelerate it to asuitable ignition speed. During the start-up mode, the alternator isoperated as a motor. During the start-up mode, the output of theinverter 17 is connected to the stator windings of the alternator 11through a start contactor 30 which is controlled by the systemcontroller 18. At the same time, a capacitor contactor 31 removes thefilter capacitors 20 from the output circuit. Because of the very highfrequencies during start-up, it is necessary to remove the filtercapacitors 20 from the stator circuits.

During start-up, DC power is drawn from a battery 33 through a fuse 34and is applied to a boost chopper 36. The boost chopper ramps thevoltage of the DC battery power from 0 to a voltage which, whenconverted to AC by the inverter 17, will drive the alternator as a motorat a speed that will enable safe ignition of the turbine. Preferably, ashaft position sensor 37 generates a signal which is applied to thesystem controller 18 which in turn uses the signal to control theinverter 17 to generate a three-phase output which commutates the statorwindings of the alternator to ramp the alternator and turbine up toignition speed.

Referring to FIG. 3, a suitable rectifier circuit is schematicallyillustrated. The three-phase stator windings 40, 41, 42, deltaconnected, are connected as illustrated by six diodes 43 a, 43 b, 43 c,43 d, 43 e, 43 f to the DC bus 16.

Referring to FIGS. 4 a and 4 b, a suitable inverter circuit isschematically illustrated. (FIG. 4 a illustrates a delta connection andFIG. 4 b a star connection for the stator winding.) The invertercomprises six solid state (IGBI) switches which, during the start-upmode, can alternately connect one corner of the delta connected statorwindings to the plus or minus side of the DC bus 16 through contactor30. Also, the solid state switches 44 a, 44 b, 44 c, 44 d, 44 e, 44 fconnect either the plus or minus side of the DC bus to the filterinductors 19 at all times and after start-up to the filter capacitors 20through contactor 31. The inverter is used to generate three-phaseoutput signals. It is capable of providing a wide variety of outputvoltages and frequencies as controlled by a microprocessor in the systemcontroller. The output inverter is used in two distinctly different waysduring start-up and power out operations of the power generation system.

During the start-up phase, the inverter is used to output time variantvoltages and frequencies needed to drive the alternator as a motor andto accelerate the alternator turbine drive shaft to rotation speedsnecessary for sustained operation of the power generation system. In itspresent configuration, this requires three-phase voltages ranging from 0up to 350 volts at frequencies from near 0 and up to 2 kHz.

During the power out phase, the inverter is used to output three-phasevoltages consistent with user power requirements. Typical voltages are480 vac, 240 vac, 208 vac, 120 vac at frequencies of 50, 60 and 400 Hz.This system is not limited to these values and a nearly infinite rangeof voltages and frequencies could be selected if desired.

Certain applications of the power generation system require the outputinverter to be capable of line tie to an existing power grid. Linephasing circuitry is used in conjunction with a system controller tomonitor the phase of the power grid voltage and synchronize the powergeneration system to it. In like manner, the system controller canmonitor power grid voltage amplitudes and adjust the power generationsystem output to facilitate and control the transfer of power to thegrid.

FIG. 5 schematically illustrates the portion of the system controllerfor generating an open loop waveform for driving the inverter 17. Afrequency generator 50 generates output pulses at frequencies selectablebetween 250 Hz and 600 kHz by a CPU 51. These pulses are applied toadvance the output in sine wave PROMs (programmable read only memories)52 a, 52 b, 52 c. The outputs from the sine wave PROMs (basically a 256Klookup table) are phase shifted from each other exactly 120° apart. Theoutputs from the PROMs are applied to digital-to-analog converters 53 a,53 b, 53 c, producing three analog sine waves. The amplitude of eachwaveform out of the digital-to-analog converters is individuallycontrolled by a sine wave (amplitude) command. The sine waves are thencompared in pulse width modulators 54 a, 54 b, 54 c with a triangle wavefrom a triangle wave generator. The frequency of the triangle wavegenerator is controllable. The pulse width modulated waveforms are thenapplied through drive select gates 55 a, 55 b, 55 c to drivers 57 a, 57b, 57 c. In the currently implemented embodiment, the drivers producethree complimentary pairs of pulse signals for controlling the inverter.The waveform generator is used to drive the inverter during the powerout mode when the turbine is driving the alternator. The waveformcircuit, so far as described, is open loop. In other words, it is notcontrolled by alternator rotation speed. However, various feedbacksignals can be used to adjust the amplitude of signals out of thedigital-to-analog converter. While the waveform circuit is principallyused to drive the inverter during the power out mode, it may be used tocontrol the inverter at the very beginning of the start-up mode to causethe armature to rotate at least once. This permits phasing of the Halleffect sensor signals.

Three Hall effect switches 58 are mounted to pick up magnetic pulses120° apart as the common shaft rotates. These signals are processed by aHall logic circuit 59 to produce a pair of signals corresponding to eachpickup pulse. The three pairs of signals are gated by the drive selectgates 55 a, 55 b, 55 c to the drivers 57 a, 57 b, 57 c. The positionsensor system consists of permanent magnets and Hall effect sensorswhich are used during turbine engine start-up to commutate electricalpower to the stator windings of the alternator. Phasing of the sensorsis accomplished at the beginning of the start-up phase by brieflyrotating the turbine alternator shaft in the direction of normalrotation. Rotation of the shaft during this initial period of thestart-up phase is accomplished by the microcomputer control of theoutput inverter system in an open loop configuration that does notutilize the Hall effect sensors. Once phasing of the sensors has beencompleted, their signals are directed to the output inverter section ofthe system to facilitate start-up of the turbine engine under closedloop control. The Hall effect pickups enable a closed loop commutationof the inverter 17 and the stator windings of the alternator. A gaincontrol circuit 61 processes feedback from the inverter circuit 17 toadjust the gain of the driver circuits to balance the output of thethree phases output from the inverter 17.

During the start-up mode, the battery supplies power to the DC busthrough the boost chopper. FIG. 6 is a schematic of a boost chopper forsupplying the DC bus with a voltage of 0 to 350 volts from a 12 or 24volt battery during the start-up mode. When the boost chopper switches65 a and 65 b are closed (conducting), current flows in an inductor 66.When the switches 65 a and 65 b are open, the magnetic field in theinductor collapses driving end A of the inducter very positive withrespect to end B and supplying current through diodes 67 a and 67 b tothe positive and negative sides of the DC bus, respectively. Theswitches 65 a and 65 b are driven at 4 kHz. The duty cycle is controlledfrom 0 to 100% enabling the output voltage across DC bus capacitors 70to vary from 0 to 350 volts. The use of a boost from 0 chopper circuitenables a gradual increase in the rotational speed of the alternatorduring start-up.

During the power out mode, the battery is charged by a charger circuit.Charger switches 68 a and 68 b are switched at about 1 kHz. The dutycycle is adjustable. When the charger switches 68 a and 68 b are closed,current from the DC bus flows through inductor 66. When the chargerswitches are opened, side B of the inductor goes positive with respectto side A and charges the battery drawing current through diodes 69 aand 69 b. It is not necessary, as illustrated here, that the boost andcharger circuits share the same inductor.

In the preferred embodiment of this invention designed for a 45 kW poweroutput, the following components are sized as set forth: filterinductors 19  300 mH per phase filter capacitors 20  100 μF per phase DCbus capacitor 70 4700 μF IGBT switches in inverter 17  400 A/600 V

FIG. 7 illustrates the interaction between the system controller and thegas turbine. The system controller utilizes three microprocessors thatcommunicate with each other through a high speed serial link and providethe following functions: (1) control of the electrical power required torotate the turbine rotor up to speeds necessary to sustain operation ofthe turbine; (2) process and control of the electrical power generatedby the alternator during power out system operation to providethree-phase output power at common line voltages and frequencies; (3)control of other subsystems needed to operate the power generationsystem, such as the ignitor, cooling fans, fuel and oil pump; (4) signalconditioning and control of instrumentation for measurement ofpressures, temperatures, flow and speed; and (5) generation and controlof a control panel providing a user interface for system operation anddiagnostics.

The three microprocessors each have their own associated memoryprogrammed to run independently. One microprocessor is directed tomonitoring the keypad, display and RS232 communicators. A secondmicroprocessor is devoted to monitoring the turbine parameters, toactuate fault trips and to log a history of operation parameters for thelatest hour of operation. The third microprocessor monitors and directsthe electrical circuit selected frequencies, voltages, actuates relays,etc.

OPERATION

There are two separate modes of system operation. In the first mode, thesystem controller 18 is used to control the boost chopper 36 and outputinverters 17 to vary the output voltage and frequency as a function oftime. Operating in this manner, the alternator is utilized as a variablespeed motor to rotate the engine at speeds required for the gas turbinesustained operation. In the second mode of operation, the invertersection is automatically reconfigured by the system controller 18 forproviding user power output. In this mode of operation, high frequencyAC power output from the alternator is converted to DC power by therectifier 15 and applied to the input of the inverter. The inverter, inconjunction with the system controller, provides the desired three-phaseoutput voltages and frequencies required in normal user applications.The output voltage frequency and phase are controlled in a mannerconsistent with stand-alone and line tie user applications.

The control panel 72 provides the interface between the user and thecontroller. It provides the user with various control andinstrumentation options, such as start-up, shut down, line tie anddiagnostics. During normal start-up and operation of the system, thesystem controller sequences and controls the power generation system asfollows.

1) On command from the control panel 72, the controller 18 sendsappropriate commands to the waveform generators and boost chopper toinitiate brief rotation of the turbines so that the Hall positionsensors are properly phased for subsequent start-up functions.

2) Next, the controller controls the boost chopper 36 and the waveformgenerator (see items 50 to 54 and 58 in FIG. 5) to ramp up three-phasevoltages and frequencies to the inverter. The three-phase outputs aredirected to the alternator which responds by accelerating the rotationof the turbine shaft to speeds necessary for its sustained operation.

3) During the above start-up sequence, the system controller monitorsand controls other functions, such as fuel flow, ignition, rotationspeeds, temperatures and pressures.

4) Following the start-up phase, the system controller reconfigures theboost chopper to operate as a battery charger. In addition, the waveformgenerator is reset to provide signals needed for generation of userpower output requirements. These signals are connected to the input ofthe selector switch where they are directed to the drivers and inverter.As a result, the inverter provides the desired three-phase outputvoltages and frequencies desired by the user.

5) During normal power out operation as described in 4) above, thesystem controller monitors and controls all functions necessary forcontrol of the power generation system including, but not limited to,control and/or monitoring of fuel flow, temperature, pressure, speed,run time and various diagnostics unique to the components of thecomplete power generation system.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is desired protected by Letters Patentis set forth in the following claims.

1. A method of controlling a turbine/alternator comprising a gas driventurbine and permanent magnet alternator on a common shaft comprising:providing electric power to said turbine/alternator through an invertercircuit to start said turbine/alternator to achieve self-sustainedoperation of said turbine/alternator; reconfiguring said invertercircuit to output electric power from said turbine/alternator whenself-sustained operation of said turbine/alternator is achieved; andsupplying the electric power to a power line while monitoring said powerline.
 2. The method of claim 1, wherein the voltage of said power lineis monitored when monitoring said power line.
 3. The method of claim 2,wherein the phase of the power line voltage is monitored when monitoringsaid power line, and the electric power to be supplied is synchronizedwith the phase of the power line voltage.
 4. The method of claim 2,wherein the amplitude of the power line voltage is monitored whenmonitoring said power line, and the electric power to be supplied isadjusted to facilitate the transfer of power to the power line.
 5. Themethod of claim 1, wherein during providing electric power to saidturbine/alternator, controlled combustion of fuel and air is provided tosaid gas driven turbine of said turbine/alternator.
 6. The method ofclaim 1, wherein when reconfiguring said inverter circuit, said invertercircuit is connected to said turbine/alternator through a rectifier. 7.The method of claim 1, wherein said inverter circuit comprises an outputfilter for filtering said electric power, and said output filter isremoved when providing electric power to said turbine/alternator throughsaid inverter circuit.
 8. An electric system for a turbine/alternatorcomprising a gas driven turbine and permanent magnet alternator on acommon shaft, said permanent magnet alternator being connectable to apower line, comprising: an inverter provided for operation of saidturbine/alternator; means to provide electric power to saidturbine/alternator through said inverter to start saidturbine/alternator to achieve self-sustained operation of saidturbine/alternator; means to reconfigure said inverter to outputelectric power from said permanent magnet turbine/alternator to supplythe electric power to said power line; and a monitor circuit to monitorsaid power line.
 9. The electric system of claim 8, wherein said monitorcircuit is adapted to monitor the voltage of said power line.
 10. Theelectric system of claim 9, wherein said monitor circuit is adapted tomonitor the phase of the voltage of said power line, and said means toreconfigure said inverter is adapted to output the electric powersynchronized to the phase of the power line voltage.
 11. The electricsystem of claim 8, wherein said monitor circuit is adapted to monitorthe amplitude of the voltage of said power line, and said means toreconfigure said inverter is adapted to adjust the output electric powerto facilitate the transfer of power to said power line.
 12. The electricsystem of claim 8, further comprising means to provide controlledcombustion of fuel and air to said gas driven turbine to achieveself-sustained operation of said gas driven turbine.
 13. The electricsystem of claim 8, wherein said means to reconfigure said inverterconnects said inverter to said turbine/alternator through a rectifier.14. The electric system of claim 8, wherein said means to reconfiguresaid inverter is adapted to supply electric power to said power line ata common line voltage and frequency.
 15. The electric system of claim 8,further comprising an output filter for filtering said electric power,which output filter is removable when said means to provide electricpower provides electric power to said turbine/alternator.